Push-Out Behavior of Rectangular Concrete-Filled Steel Tubes

A series of push-out tests of rectangular concrete-filled tubular columns (CFT) was recently conducted. The objective of this research program was to identify the shear transfer mechanisms between the infilled concrete and the steel tube and to determine a method for evaluating the capacity of the steel-concrete interface in a CFT column. The experimental variables investigated were the wall slenderness ratio (b/t) of the steel tube and the use of shear tab connections to apply axial load to the steel tube. The results of this study indicated that three mechanisms are responsible for shear transfer along the steel-concrete interface in a push-out specimen: adhesion to the concrete to the steel surface, friction, and wedging of the concrete core. The role of each mechanism in transferring shear between the concrete and steel in the CFT push-out specimen at various stages of load and slip is discussed. Design guidelines for shear transfer in rectangular CFT columns are presented, including a proposed bond strength equation and a recommended strength reduction factor for bond.